Since this just cropped up, it sounds like someone edited something along your preferred route in the map editor & it was part of the map update that was pushed out early last week.
Take a look at the map editor (and read the Wiki) to see if you can find the problem. A good indicator to drive your preferred route and when the app stops giving you wrong directions and puts you back on your preferred path. The error is usually close to that point.
(This is a slightly modified copy and paste of my post in this thread, which you also may find useful: http://www.waze.com/forum/viewtopic.php?f=144&t=19628)

